The invention relates to a method for smoothening a web in a papermaking operation before it is coated. The steps in the smoothening operation include calendering a moistened web in a heated hard nip. This process improves the receptivity of the web surface to coating applications, and to subsequent printing operations.
It is known that an otherwise untreated web of paper or paperboard rawstock will react adversely to the application of a liquid by becoming rougher, since the surface fibers of the web absorb moisture leading to fiber swelling, breaking of hydrogen bonds and fiber reorientation. If the liquid application is in the form of a coating or the like, the web may actually become smoother because of the presence of smoothing ingredients in the coating, but the improvement in smoothness is not as great as would be expected because the surface of the rawstock beneath the coating becomes roughened in the presence of the liquid phase of the coating. Because of this phenomenon, and because one of the objectives in the production of paper and paperboard is to produce a smooth finish, it is desirable to smoothen the surface of the rawstock web before coating.
One of the known techniques for smoothing the surface of a web of paper or paperboard rawstock before coating is to pre-calender the web. This may be accomplished on a paper machine after the web is dried. In the pre-calendering step, the web is passed through one or more nips between the rolls of a calender, the nips providing a degree of compression to the paper surface to flatten the surface fibers, thereby creating a smoothened surface. The degree of finish obtained by this method depends upon the nip pressure reached in the calender. However, the smoothness obtained by pre-calendering a dried web is short lived when the web is subsequently brought into contact with water or a composition containing water, since dry cellulose fibers will not bond under pressure. On wetting, there will be substantial reorientation of the fibers at the web surface due to fiber swelling and release of stress. An improvement that alleviates this unwanted result is to moisten the dry web before pre-calendering. The compressing of moistened cellulose fibers will consolidate the web without undue stress, and the smoothened surface that is achieved will retain a substantial amount of its integrity for a longer time after wetting than is common for a web that is compressed in an essentially dry state.
Conventional processes for wet calendering employ either a water box on the calender, water sprays, or steam showers. In most cases, the quantity of moisture transferred to the web by these means is uneven, too great or too small, resulting in unsatisfactory results. Nevertheless, recent innovations in the paper industry have provided equipment that is capable of applying controlled amounts of water to a web in a uniform manner, particularly as disclosed in pending U.S. patent application Ser. No. 09/296,394, owned by the present assignee herein. The ability to apply a controlled amount of moisture to the web at the pre-calender yields better results. Nevertheless, there remains a need in the art to further enhance the process, and, accordingly, the present invention is designed to advance the process efficiency and product quality that can be achieved in a pre-calendering operation.
The importance of having a smooth rawstock surface and its effect on the printability of coated paper is well known. Pigmented coatings tend to hide or cover up some of the surface imperfections in a paper rawstock. However, the effectiveness of a coating in smoothing the surface of a paper web depends upon the condition of the surface on which the coating is applied. The effect on paper due to the application of a coating is discussed in the article entitled xe2x80x9cWater-Paper Interaction During Paper Coatingxe2x80x9d, by J. Skowronski and P. Lepoutre, published in the November 1985 issue of TAPPI Journal, at pages 98-102. Meanwhile, the effect of pretreatment or pre-calendering of a paper product before coating is discussed in the articles xe2x80x9cEffect of Pretreatment of LWC Basestock on Coated Paper Propertiesxe2x80x9d, by P. Lepoutre, W. Bichard and J. Skowronski, published in the December 1986issue of TAPPI Journal, at pages 66-70, and in the article entitled xe2x80x9cPrecalendering and its Effect on Paper-Coating Interactionxe2x80x9d, by G. Engstrom and J. F. Lafaye, published in the August 1992 issue of TAPPI Journal, at pages 117-122.
Likewise, the effect of applying moisture to a paper web before the web enters a calender nip, and various methods for making water application are also known, and are shown, for example, in U.S. Pat. Nos. 5,378,497; 5,505,820; 5,522,312; and 5,607,553, all owned by the present assignee herein. However, none of these prior art references deal with the concept of smoothening the web while reducing loss of bulk by drying the web to a very low moisture content before the application of moisture, passing it through a hard calender nip, and heating at least one roll of the calender nip as disclosed herein. Nor do the prior art references teach the use of the heated roll to dry the applied moisture as the web exits the calender nip as disclosed herein, especially in connection with a pre-calendering treatment on paper machine.
When a web of paper rawstock is exposed to water during a coating process, the surface beneath the coating layer becomes roughened as described above. In order to offset this increase in roughness, the present invention is directed to a process whereby the paper rawstock is pre-calendered before coating on the paper machine. More particularly, the present invention is directed to a process whereby a thin film of liquid is uniformly applied to the surface of the rawstock web using an apparatus particularly as disclosed in U.S. patent application Ser. No. 09/296,394, which is herein incorporated by reference. The application of a thin film of liquid to the paper surface allows only the surface fibers of the web to be plasticized, so that, upon pre-calendering in a heated, hard nip formed, for example, by a pair of steel rolls, the surface of the web is fixed. At the same time, loss of bulk of the web is reduced because it does not become wetted throughout with the liquid. According to the present invention, the paper surface may be characterized as being xe2x80x9cfixed,xe2x80x9d in that it is substantially irreversibly smoothened by practicing the process described herein. Because of the fixed surface, subsequent coating application will have a minimal impact on re-roughening the surface of the web. In the preferred method of the invention, the web enters the calender nip prior to the time that the liquid application has a chance to completely penetrate the full thickness of the web. The liquid used to moisten the paper surface can be plain water, or a solution or dispersion of a binder, thickener, or one or more other functional additives, such as lubricants or surface-active agents, which may be heated or at ambient temperature.
Another benefit of using the device of the present invention is that it eliminates the presence of a standing pond of water or finishing liquid on or adjacent to the heated roll, and, as a result, the inherent cooling effect that would be associated with such an arrangement is eliminated. Because the pond is removed, maintaining a roll temperature above the boiling point of water then becomes practical, and operating temperatures in excess of 212xc2x0 F. (100xc2x0 C.) are possible. These higher temperatures in turn are capable of producing even greater smoothening results. In this respect, the temperature of the heated roll can range up to about 400xc2x0 F. or higher, for example, from about 300xc2x0 F. to about 500xc2x0 F., and is limited only by the potential for causing adverse heat effects, such as charring, on the substrate being processed.
The present invention may be practiced on any grade of paper or paperboard that contains moisture sensitive fibers. These include bleached, unbleached, or recycled paper and paperboard. The web may thus consist solely of cellulose fibers, a mixture of cellulose and synthetic fibers, or paper filled with mineral or organic pigments. The web can be unsized or sized in the furnish and/or on the surface, provided the water sensitivity of the fibers is retained. Before pre-calendering, the web is preferably dried to a very low moisture content (e.g., on the order of less than 5% and most preferably, between 1-2%) before application of the water or finishing liquid.
The pre-calendering step may take the form of a typical calendering operation including one or more pairs of nipped rolls loaded to produce a nip pressure on the order of about 400-7000 psi. The nipped rolls are preferably fabricated from steel. In a preferred embodiment of the invention, the pre-calendering step comprises a heated nip formed between two rolls, at least one of which is heated to a surface temperature greater than about 300xc2x0 F. In this regard, the heated nip may be formed between one heated and one unheated roll or it may be formed between two heated rolls in the calender stack. The moisture application is made to the surface of the web that contacts the heated roll. One nip with controlled loading force will normally suffice, but multiple nips, in tandem or stacked, may preferably be employed.
The passage of the web through the heated nip partially dries the moistened web. Additional drying may be effected by at least partially wrapping the web, after it exits the heated nip, over the surface of at least one heated roll. In this regard, the heated roll may be the same roll forming the at least one heated nip, or it may be another heated roll in the calender configuration. In a preferred embodiment the web is partially wrapped around the heated roll forming a heated nip as the web exits the heated nip. This step allows the heat from the roll surface to re-dry the web before it is passed to the coaters.
With the foregoing in mind, it is an object of the present invention to provide an improved method for pre-calendering a paper web, which, as alluded to herein, may be a paper or paperboard web, so that the web will not be affected by a subsequent coating application. The invention therefore comprises, in one embodiment, a method of pre-calendering that generates a high level of surface smoothness, and which is essentially resistant to reversion upon the subsequent application of a water based coating composition, comprising:
a) preparing a paper web on a paper machine;
b) drying the web to a moisture content within the range of from about 0.5-5%;
c) directing the dried web through a pre-calender device located on the paper machine, said pre-calender device consisting essentially of a stack of adjacently placed hard rolls forming a plurality of nips, at least one of said hard rolls being heated;
d) moistening at least one surface of the web by applying water or an aqueous liquid in the form of a thin film to the web just before the web enters a heated nip formed by a heated roll such that the wet surface of the web contacts the heated roll;
e) passing the web through the heated nip to smoothen the moistened surface while substantially retaining the bulk of the web; and,
re-drying the web.
In another aspect, the invention comprises a paper machine for manufacturing paper or paperboard, comprising: a calender stack formed from a stack of adjacently placed hard rolls, at least one of which is heated, each pair of adjacently placed hard rolls forming a nip, the at least one heated roll forming a heated nip; and a moisture application means positioned proximal to at least one heated roll to deliver a thin film of water or finishing fluid to the surface of the web before it enters the heated nip.